To anyone who didn't know/is surprised, where have you been?
Everything is tied to the NNID, which is tied to the console, and every account on that console is able to access all of the console's content.
I'm pretty sure Nintendo realized this was going to happen but didn't care.

This sort of thing was awful when the Wii was released. That they went with a similar system for Wii U is unfathomable. Do these people even use the internet?